1. Field of the Invention
The present invention relates to network monitoring device and method, and more particularly, to network monitoring device and method for monitoring load on a network.
2. Description of the Related Art
To an OSI (Open Systems Interconnection)-based or TCP/IP (Transmission Control Protocol/Internet Protocol)-based network are connected various network devices such as a SONET (Synchronous Optical Network) or SDH (Synchronous Digital Hierarchy) transmission device, an ATM (Asynchronous Transfer Mode) exchange, a router and terminal units. Such network devices connected to the network are called nodes.
FIG. 17 shows an example of a conventional network configuration. In FIG. 17, an OSI network is illustrated by way of example. The OSI network comprises a plurality of areas 910, 920 and 930. The area 910 includes nodes 911 and 912. The area 920 includes nodes 921 to 923, and the area 930 includes nodes 931 and 932.
In the OSI network, each of the nodes 911, 912, 921 to 923, 931 and 932 transmits and receives Link State PDU (Protocol Data Unit) (hereinafter referred to as LSP) to and from another node in the same or other areas 910, 920 and 930 (level-1 LSP for intra-area transmission and level-2 LSP for inter-area transmission) by means of Network Layer CLNP (Connection Less Network Protocol). The LSP includes routing information, which is information indicating routes which are to be followed for appropriate transfer of packets. As the LSP is transmitted and received, the routing information is dynamically exchanged between the nodes 911, 912, 921 to 923, 931 and 932.
In the TCP/IP network, on the other hand, using IP (Internet Protocol) or ARP (Address Resolution Protocol), individual nodes exchange a Hello PDU or ARP packet with each other. The Hello PDU and ARP packet include routing information, whereby the routing information is dynamically exchanged also in the TCP/IP network.
The routing information exchanged in the OSI or TCP/IP network is loaded, as a routing table, into the memory of a node (router) that takes care of routing. In the routing table are registered, with respect to each destination node, an identifier of a communication interface for communicating with the destination node, an identifier (e.g., address) of a router (or gateway) which relays packets addressed to the destination node, etc. Thus, the routing table is stored in routers, so that each router can identify a destination to which a received packet is to be transferred. As a consequence, packets on the network can be passed on and transferred to correct destination nodes by the routers.
Meanwhile, a network has its own limit on the number of nodes up to which packets can be correctly routed.
For example, the router is unable to store a routing table that exceeds the capacity of the built-in memory thereof. Thus, the router has its own limit on the number of destination nodes which the router can identify, and therefore, cannot correctly transfer packets to all of nodes exceeding the limit. The number of identifiable nodes differs from router to router.
If there exists even a single router which can identify fewer nodes than the number of nodes connected to the network, then inter-node communications of the network cannot be guaranteed. When configuring a network, therefore, the total number of nodes needs to be restricted to such an extent that a router with the smallest number of identifiable nodes can correctly perform routing.
Also, since the routing information is exchanged between nodes at regular intervals, increase in the number of nodes connected to the network inevitably entails an increase of communication traffic of the transmission path. If the communication traffic exceeds the transmission capacity of the transmission path, appropriate transmission and reception of packets cannot be guaranteed. Accordingly, the number of nodes which communicate via each transmission path needs to be restricted to a number falling within an allowable range of the capacity of the transmission path.
Thus, when configuring or reconfiguring a network, the user must carry on the work with a good understanding of the number of nodes up to which packet routing can be guaranteed.
In the case of a large-scale network, however, it is difficult to correctly determine the number of nodes up to which packet routing can be guaranteed, by investigating all network environments. For example, when the network is to be extended, the routing tables of all routers on the network may be looked up to determine whether the number of nodes exceeds the limit or not, but such work is extremely complicated and consumes much labor and time.
In cases where one node is added to a network, the network environment around the node to be added may simply be examined. However, where there arises a need for a substantial change of the network configuration or the number of nodes, for example, where independently operated networks are connected together, all network environments need to be examined, requiring considerable time and labor.